I currently use Paint.NET to make my skins. I've found certain textures tend to get the red/pink glitch when saved as png. These textures include human skin, titan body, and hair (maybe others, not sure at the moment). I've actually found a solution to this problem that doesn't require opening it in another program. As long as there is even a small bit of transparency on these textures, they seem to work correctly, so try making a small transparent dot in the corner of the texture (or on any part that doesn't actually show up on the model). It seems to solve the problem all the time for me. Every time the texture appears red/pink, I just add the transparent bit in the corner and it works.